Effect of heat temperature on molecular weight and thermal properties of poly(3-hydroxybutyrate-co-3hydroxyvalerate) with different comonomer contents

Polyhydroxyalkanoates (PHA) are regarded as promising bio-based and biodegradable polymers for the development of sustainable products, and their presence in the plastics industry is steadily increasing. The study of their thermal transitions and properties as a function of thermal treatment is crucial from a processing perspective, as well as for the optimisation of material performance. This study investigates the effects of 3-hydroxyvalerate (3HV) content (0, 8, 36, 48, 57, 60 %.mol) and heat treatment on the thermal behaviour of poly(3-hydroxybutyrate- co -3-hydroxyvalerate) (PHBV) using modulated differential scanning calorimetry (MDSC). Thermal stability, evaluated by gel permeation chromatography (GPC), showed that polyhydroxybutyrate (PHB) appears to have a lower thermal stability than PHBV with 8 mol.% 3HV but higher 3HV content tends to reduce stability, reflected by greater molecular weight loss with increasing heating temperature. Molecular weight decreased linearly for all samples. Self-nucleation enhanced crystallisation occurred during cooling, in most PHBV, even at high 3HV content. A melt memory effect persisted even above the melting of all the residual crystallites, and it promoted PHAs crystallisation. Once thermal history was fully erased, crystallisation temperature became independent of heat treatment, indicating that crystallisation kinetics depended more on thermal history than degradation. Melting temperature remained stable for some PHAs but decreased for others. It appears that thermal transitions, properties and their evolution with heating conditions are specific to each PHA. MDSC revealed complex melting behaviour involving reversing and non-reversing processes. Lower 3HV contents and fully released thermal history led to more significant structural rearrangements, including cold crystallisation and melting–recrystallisation during the heating scan. Self-nucleation process and cold crystallisation appear to promote the melting of formed crystallites through non-reversing mechanisms, whereas, with the removal of thermal history and increased thermal degradation promote the melting of crystallites via reversing mechanisms.